• An anabolic, endergonic, carbon dioxide (CO₂) requiring process that uses light energy (photons) and water (H₂O) to produce organic macromolecules (glucose).

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​

​

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

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• Almost all plants are photosynthetic autotrophs, as are some bacteria and protists

• Autotrophs generate their own organic matter through photosynthesis
• Sunlight energy is transformed to energy stored in the form of chemical bonds

(a) Mosses, ferns, and

flowering plants

(b) Kelp

(c) Euglena

(d) Cyanobacteria

THE BASICS OF PHOTOSYNTHESIS

Light Energy Harvested by Plants & Other Photosynthetic Autotrophs

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

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WHY ARE PLANTS GREEN?

Plant Cells have Green Chloroplasts

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The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids).

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Photosynthesis occurs in chloroplasts

• In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts
• A chloroplast contains:
• stroma, a fluid
• grana, stacks of thylakoids
• The thylakoids contain chlorophyll
• Chlorophyll is the green pigment that captures light for photosynthesis

• The light reactions convert solar energy to chemical energy
• Produce ATP & NADPH

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​

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• The Calvin cycle makes sugar from carbon dioxide
• ATP generated by the light reactions provides the energy for sugar synthesis
• The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose

Light

Chloroplast

Light

reactions

Calvin

cycle

NADP⁺

ADP

+ P

AN OVERVIEW OF PHOTOSYNTHESIS

Chloroplast Pigments

• Chloroplasts contain several pigments

• Chlorophyll a
• Chlorophyll b
• Carotenoids
• Xanthophyll

Figure 7.7

• Chl a has a methyl group

• Chl b has a carbonyl group

• Chloroplasts absorb light energy and convert it to chemical energy

Light

Reflected

light

Absorbed

light

Transmitted

light

Chloroplast

THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED

• Two main parts (reactions).

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1. Light Reaction or

Light Dependent Reaction

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Produces energy from solar power (photons) in the form of ATP and NADPH.

• Occurs in the Thylakoid membranes

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• During the light reaction, there are two possible routes for electron flow.

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A. Cyclic Electron Flow

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B. Noncyclic Electron Flow

Steps of Photosynthesis

• Light hits reaction centers of chlorophyll, found in chloroplasts

• Chlorophyll vibrates and causes water to break apart.

• Oxygen is released into air

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• Hydrogen remains in chloroplast attached to NADPH
• “THE LIGHT REACTION”

Cyclic Photophosphorylation

• Process for ATP generation associated with some Photosynthetic Bacteria
• Reaction Center => 700 nm

• Occurs in the thylakoid membrane.
• Uses Photosystem I only
• P700 reaction center- chlorophyll a
• Uses Electron Transport Chain (ETC)
• Generates ATP only

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​

Noncyclic Photophosphorylation

• Photosystem II regains electrons by splitting water, leaving O₂ gas as a by-product

• Occurs in the thylakoid membrane

​

• Uses PS II and PS I

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• P680 rxn center (PSII) - chlorophyll a

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• P700 rxn center (PS I) - chlorophyll a

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• Uses Electron Transport Chain (ETC)

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• Generates O₂, ATP and NADPH

• ADP + → ATP

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• NADP⁺ + H → NADPH

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• Oxygen comes from the splitting of H₂O, not CO₂

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H₂O → 1/2 O₂ + 2H⁺

(Oxidized)

In the light reactions, electron transport chains generate ATP, NADPH, & O₂

• Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons
• The excited electrons are passed from the primary electron acceptor to electron transport chains
• Their energy ends up in ATP and NADPH

How the Light Reactions Generate ATP and NADPH

2 H⁺ + ¹/₂

Water-splitting

photosystem

Reaction-

center

chlorophyll

Light

Primary

electron

acceptor

Energy

to make

Electron transport chain

Primary

electron

acceptor

Primary

electron

acceptor

NADPH-producing

photosystem

Light

NADP⁺

1

2

3

• The production of ATP by chemiosmosis in photosynthesis

Thylakoid�compartment�(high H⁺)

Thylakoid�membrane

Stroma�(low H⁺)

Light

Antenna�molecules

Light

ELECTRON TRANSPORT �CHAIN

PHOTOSYSTEM II

PHOTOSYSTEM I

ATP SYNTHASE

Summary—Light Dependent Reactions

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a. Overall input

light energy, H₂O.

b. Overall output

ATP, NADPH, O₂.

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Steps of Photosynthesis

• The DARK Reactions= Calvin Cycle

• CO2 from atmosphere is joined to H from water molecules (NADPH) to form glucose

• Glucose can be converted into other molecules with yummy flavors!

Summary—Light Independent Reactions

a. Overall input

CO₂, ATP, NADPH.

b. Overall output

glucose.

Review: Photosynthesis uses light energy to make food molecules

• A summary of the chemical processes of photosynthesis

Light

Chloroplast

Photosystem II�Electron transport chains Photosystem I

CALVIN CYCLE

Stroma

Electrons

LIGHT REACTIONS

CALVIN CYCLE

Cellular respiration

Cellulose

Starch

Other organic compounds